1. Field of the Invention
The present invention relates to an in-plane switching liquid crystal display device, which carries out display by controlling the alignment direction of liquid crystal molecules with an in-plane electric field parallel to a substrate surface.
2. Description of the Related Art
Due to the recent rapid technical advancement on liquid crystal display devices, there is a strong demand of achieving a video performance for liquid crystal display devices, which is as high as the video performance of CRT (Cathode-Ray Tube) display devices. Because an IPS (In-Plane Switching) liquid crystal display device, in particular, has an excellent characteristic of achieving an extremely wide viewing angle, its adaptation to TVs is rapidly spreading. The IPS liquid crystal display device is therefore demanded of an improvement on the response performance to adequately display moving pictures, or video images.
Generally, a response speed τ (=τon+τoff), which is the sum of a response time τon for a liquid crystal to switch from black display to white display and a response time τoff for the liquid crystal to switch from white display to black display, is used as an index for the response performance. The relationship among the response speed τ of an IPS active matrix type liquid crystal display device and an intersubstrate gap d and the rotational viscosity coefficient γ1 of the liquid crystal is given by the following expression 1.τ∝d2×γ1  (Expression 1)
As apparent from the expression 1, to make the response of the IPS liquid crystal display device faster, or reduce the response speed τ, it is effective to reduce the intersubstrate gap d, or narrow the gap, and reduce the rotational viscosity coefficient γ1 of the liquid crystal.
There is a liquid crystal display device, which is designed to achieve a fast response by defining the rotational viscosity coefficient γ1 of the liquid crystal and the refractive index anisotropy Δn of the liquid crystal (see, for example, Japanese Patent Laid-Open Publication No. 2000-310797). Japanese Patent Laid-Open Publication No.2000-310797 describes that the response speed τ can be improved to 17×10−3 sec or less on one side including the half tone response by setting (γ1/Δn2) to 6.0 Pa·sec or less. The liquid crystal display device described in the publication achieves the improved response speed by setting the intersubstrate gap d to 3 μm and using a liquid crystal material in which the amount of the neutral material contained in the liquid crystal material is set to 40 wt % or greater and the rotational viscosity coefficient γ1 is set to an extremely small value of 45 to 55 mPa·sec.
There is another IPS liquid crystal display device whose response speed is improved by setting the product of the intersubstrate gap d and the refractive index anisotropy Δn of the liquid crystal (=d×Δn) in a range of 0.2 to 0.55 μm (Japanese Patent Laid-Open Publication No. H9-297306).
The prior arts however have the following shortcomings. First, the neutral material used in the liquid crystal display device described in Japanese Patent Laid-Open Publication No. 2000-310797 has a high vapor pressure as compared with other components of the liquid crystal layer. When a liquid crystal composition containing a large amount of the neutral material is sealed into a cell in vacuum as in the ODF (One Drop Fill) system and the injection system, therefore, the neutral material is volatilized during sealing. Volatilization of a large amount of the neutral material not only changes the characteristic of the liquid crystal composition but also changes the concentration, resulting in crystallization of a liquid crystal component. If the liquid crystal composition has a large content of a single component, the compatibility of the individual components is reduced. Further, to acquire the proper dielectric anisotropy Δ∈ of the liquid crystal while increasing the ratio of the neutral material, a material having a large dielectric constant and a strong polarity should be added as a liquid crystal component. Because most of such materials have high affinity to ions of Na or so, there is a possibility of generating spots, making the materials extremely difficult to use.
The angle defined by the rubbing direction and the in-plane electric field is set to 88° in the liquid crystal display device described in Japanese Patent Laid-Open Publication No. H9-297306. With such an angle, however, the torque that is applied to the liquid crystal aligned in the initial alignment direction the instant the electric field is applied is small so that fast driving cannot be achieved.